1. Field of the Invention
The present invention belongs to the technical field of information recording media, such as optical disks capable of high-density recording or high-capacity recording, and the technical field of information reproducing apparatuses, information recording apparatuses, and information recording/reproducing apparatuses, capable of information reproduction and information recording from and on the information recording media.
2. Related Background Art
Conventionally, the development of information recording media for high-density information recording and mass information storage, and the development of information recording apparatuses and reproducing apparatuses used with the information recording media have been underway. Especially, with the development of information society, not only the amount of information processed but also mobile applications or portable use increase to increase the demand for compact, high-capacity information recording media. Under these circumstances, use of relatively easy-to-handle optical disks for optical recording and reproduction, such as CD-ROM (Compact Disc-Read Only Memory), MD (Mini Disc), and DVD-ROM (Digital Versatile-Read Only Memory), is increasing.
The recording density or recording capacity of these optical disks is determined by certain factors. The factors include kind of light used for recording and reproduction, for example wavelength of semiconductor laser, NA (Numerical Aperture) of an optical system practically usable for the wavelength, shape and depth of a depression or projection, or pit, corresponding to recorded information, and quality of an error signal obtained from the pit for control of the apparatus.
For example, for the conventional optical disk, the depth of a groove of a track or the depth of a pit is set to λ/4n at which a signal to be reproduced as information from the pit or RF signal rises to its maximum level. On the other hand, a push-pull signal as an error signal for servo application cannot be detected at λ/4n, and rises to its maximum level at λ/8n. In view of the above two conditions, a value close to λ/6n has been used for the actual depth of a pit. It should be noted that λ is the wavelength of laser and n is the refractive index of a recording medium used.
However, according to the research by the inventors, when an optical system, for example with a NA of 0.75 or greater, that is, with a high NA lens is adopted as the reproduction optical system in an optical disk apparatus in order to increase the recording density, optical disks with conventional pit depths (close to λ/6n) cause a technical problem of making it difficult to generate both the RF signal and the push-pull signal efficiently. Especially, when blue light laser with a short wavelength is used as the light source, the conventional optical system with an NA lens of less than 0.75 and the optical disk with the conventional pitch depth (close to λ/6n) make it difficult to generate both the RF signal and the push-pull signal efficiently as signals of great magnitude.